The present invention relates to a variable venturi-type carburetor.
A variable venturi-type carburetor normally comprises a suction piston, a needle supported by the suction piston, a fuel passage extending in the axial direction of the needle so that the needle is able to enter into the fuel passage, and a metering jet situated in the fuel passage and interacting with the needle. The fuel passage, located upstream of the metering jet, is connected to the float chamber of the carburetor. In order to obtain stable engine idling, the conventional variable venturi-type carburetor has a slow fuel system comprised of a slow fuel port continuously open to the intake passage located downstream of the throttle valve of the carburetor, a slow fuel passage branched off from the fuel passage upstream of the metering jet and connected to the slow fuel port, and a slow fuel jet situated in the slow fuel passage. In this slow fuel system, a vacuum, produced in the intake passage, acts on the slow fuel passage via the slow fuel port. Fuel in the fuel passage is sucked into the slow fuel passage via the slow fuel jet by this vacuum and fed into the intake passage from the slow fuel port. However, in a conventional variable venturi-type carburetor, if the throttle valve is abruptly opened to accelerate the engine, since the level of the vacuum acting on the slow fuel port abruptly becomes small, the level of the vacuum in the slow fuel passage also drops abruptly, and thus the amount of fuel sucked into the slow fuel passage from the fuel passage is abruptly reduced. As a result, since the amount of fuel fed into the intake passage from the slow fuel port is reduced, the fuel mixture fed into the cylinder of the engine becomes lean, and thus problems occur in that it is impossible to obtain good engine acceleration, and that the exhaust emission quality will deteriorate.